Pool Paradise

After buying the Hidden Cave mini-game at the store, go to the
practice table. Start a game, and point towards where the hidden
cave is supposed to appear. From the break position, go to the right
at about the 2 O'clock position and point at the cave located there.
If done correctly, you will be able to select the map in the
following step. Press Start to pause game play, then select the
"Inventory" option followed by "Sub Games". Now, select the Hidden
Cave map. Answer "Yes" when prompted to quit the practice game. You
should now be at the Hidden Cave, with a treasure chest with money.
After reaching it the 1st time, it becomes unlocked at the main
menu at the island rock.